Fascinating. My first thought was 'but where would the water go', but of course he explains - underground river.
My local canal had a' between the locks' drain a couple of years ago for emergency repairs to a gate. there was no plughole, but they did let the public down into the cut for a looksee. Fascinating stuff. 300 year old bricks that are usually under water. Much of it was concrete, emergency repairs from a ww2 bombing that had to be done quickly to keep the coal moving along the canals.
The new lock gates cost something like £16,000 each, and had to be made in the traditional style, and of english oak, because although the canal is functional its also heritage. The whole process cost around £50,000 iirc, what with the pumping the water and the boat and crane to remove and replace the gates, and the stunning and relocating the fish. All for a stretch of water maybe 25 metres long.
Sadly I can't find my photos of it.